The invention relates to a TDMA (Time Division Multiple Access) method, transmitter and receiver. In particular, the present invention relates to a digital mobile communication system, such as the GSM system (Groupe Special Mobile) or the DCS (Digital Cellular System).
In the TDMA class of digital mobile communication systems, several users share one carrier. Each user is allocated a separate time slot on that carrier, which is used to transmit and receive short bursts of data packets separated by a guard time. The number of users on one carrier is limited to the number of slots allocated to that particular carrier.
The GSM system is described e.g. in xe2x80x9cThe GSM System for Mobile Communicationsxe2x80x9d, M. Moyly and M.-B. Pautet, Palaiseau, France, 1992, ISBN: 2-9507190-0-7 and is therefore not described herein in geater detail.
In current GSM systems, as for example described in document WO-A-95/28771, a predetermined bit sequence in the middle of the data in a TDMA burst is used as a training sequence required in order to calculate parameters of an impulse response of a respective channel, i.e. estimated channel impulse response taps. These parameters are used for removing interactions caused by multipath transmission.
Currently, approximately five taps of the channel impulse response can be measured by the available training sequences. If longer impulse responses should be measured, a longer training sequence would be required which would decrease the amount of data within the TDMA burst.
Regarding the problem of reduced transmission efficiency associated with the use of longer training sequences, document EP-A-0 355 587 suggests to delete training preambles within a TDMA burst and, instead, to store baseband quadrature signal components as sampled data of an entire slot in delay buffers while the clock and carrier phases are estimated. However, in this case, complex signal processing circuits are necessary for obtaining the required estimations.
It is an object of the present invention to provide an improved TDMA method, transmitter and receiver, by means of which longer impulse responses can be measured.
This object is achieved by a TDMA method, comprising the steps of modulating a training sequence and generating a TDMA burst by superposing the modulated training sequence upon a data signal.
Furthermore, this object is achieved by a transmitter for a TDMA system, comprising modulating means for modulating a training sequence, superposing means for generating a TDMA burst by superposing the modulated training sequence upon a data signal, and transmitting means for transmitting said TDMA burst.
Additionally, this object is achieved by a receiver for a TDMA system, comprising receiving means for receiving a TDMA burst, extracting means for extracting a superposed training sequence from the received TDMA burst, and cancelling means for cancelling the superposed training sequence before data detection.
Since the training sequence is superposed on the TDMA burst, a long training sequence which may be as long as the whole burst may be used. Thus, the training sequence is always available and can be used to measure longer or shorter channel impulse responses.
Another advantage is that this type of training sequence could be used for estimation of frequency error as well as any change in the channel impulse response over the burst.
A preferred embodiment of the method, transmitter and receiver of the invention is evident from the attached dependent claims.